CHEM 346 – Organic Chemistry I – Fall 2014 – Quiz #3 – Solutions Key Page 1 of 8 CHEM 346 – Organic Chemistry I – Fall 2014 Instructor: Paul Bracher Quiz #3 Due: Monday, October 13th, 2014 6:00 p.m. (in Monsanto Hall 103) Student Name (Printed) Solutions Student Signature N/A Instructions & Scoring  Please write your answers on the official an‐
swer sheet. No answers marked in this booklet will be graded. You must submit a hard copy of your answer sheet. Answer sheets submitted electronically will not be graded.  You may use any resources you wish and col‐
laborate with others.  Any questions should be posted to the Black‐
board discussion board so all students have equal access to the information. Problem I II III IV TOTAL Points Earned  Your quiz answer sheet may be photocopied. Questions, Required Information, Supplementary Information Points
Available 35 26 27 12 100 CHEM 346 – Organic Chemistry I – Fall 2014 – Quiz #3 – Solutions Key Page 2 of 8 Problem I. Multiple choice (35 points total; +5 points for a correct answer, +2 points for an answer intentional‐
ly left blank, and 0 points for an incorrect answer). For each question, select the best answer of the choices given. Write the answer, legibly, in the space provided on the answer sheet. B (1) ___________ Which of the following types of reactions is least likely to occur when compound A is treated with sodium methoxide (Na+−OCH3)? (a) SN1 (b) SN2 (c) E1 (d) E2 (2) B ___________ What is the best name for compound B? (a) (2S,5S)‐5‐bromoethyl‐2‐chloromethylheptane (b) (2R,5S)‐7‐bromo‐1‐chloro‐5‐ethyl‐2‐methylheptane (c) (2R,5R)‐7‐bromo‐1‐chloro‐5‐ethyl‐2‐methylheptane (d) (3S,6R)‐1‐bromo‐7‐chloro‐3‐ethyl‐6‐methylheptane (e) (3R,6R)‐1‐bromo‐7‐chloro‐3‐ethyl‐6‐methylheptane CHEM 346 – Organic Chemistry I – Fall 2014 – Quiz #3 – Solutions Key (3) C ___________ Page 3 of 8 What term best describes the relationship of the molecules drawn below as Newman projections C and D? (a) enantiomers (b) diastereomers (c) identical compounds (d) structural/constitutional isomers (e) none of the above (4) A ___________ How many stereoisomers exist of compound E (including E itself)? (a) 3 (b) 4 (c) 14 (d) 15 (e) 16 CHEM 346 – Organic Chemistry I – Fall 2014 – Quiz #3 – Solutions Key (5) E ___________ Page 4 of 8 What is the major product when (3R,4S)‐4‐ethyl‐3‐iodooctane is treated with sodium methoxide (NaOCH3) in methanol (CH3OH)? (6) D ___________ How many steps (i.e., transition states) appear in the mechanism for the following transformation? (a) 0 (b) 1 (c) 2 (d) 3 (e) 4 CHEM 346 – Organic Chemistry I – Fall 2014 – Quiz #3 – Solutions Key (7) C ___________ Page 5 of 8 Which of the following isomers of C6H12 would release the most heat when subjected to complete combustion in an oxygen atmosphere to produce 6 equivalents of CO2 and 6 equivalents of H2O? CHEM 346 – Organic Chemistry I – Fall 2014 – Quiz #3 – Solutions Key Page 6 of 8 Energy
Problem II. Reaction Diagram (26 points). (1) (18 points) Draw two superimposed reaction diagrams on the (same) set of axes found on your answer sheet. First, plot the one‐step conversion of molecule F to molecule G in an exothermic process. On the same figure, plot a reaction diagram for the conversion of F to G when catalyzed by X. The catalyzed process pro‐
ceeds via two intermediates: first, X–F and then, X–G. The formation of X–G from X–F is the rate‐determining step of the catalyzed reaction. Label F, G, X–F, X–G, and ΔG on your plot. X–G
X–F
X+F
G°
X+G
Reaction Coordinate
(2) (8 points) A sample reaction mixture at equilibrium contains 2.8 mmol of G and 0.40 mmol of F. What is the value of ΔG° for the reaction in kJ/mol? Show your work for this calculation. ΔG° = −RT ln K = −RT ln ([G]/[F]) = −(8.315 × 10−3 kJ∙K−1∙mol−1)(298 K) ln (2.8 mmol/0.40 mmol) = −(8.315 × 10−3 kJ∙K−1∙mol−1)(298 K) ln (2.8 mmol/0.40 mmol) = −(2.4779 kJ∙mol−1) ln (7) = −(2.4779 kJ∙mol−1) ln (7) ΔG° = −4.8 kJ∙mol−1 CHEM 346 – Organic Chemistry I – Fall 2014 – Quiz #3 – Solutions Key Page 7 of 8 Problem III. Explanations (27 points). For each question posed below, write the letter of your answer in the box on the answer sheet and provide a brief explanation (of no more than four sentences) for your choice. You should draw out any relevant structures or diagrams in your explanation. (1) (9 points) Of compounds H and J, which reacts faster with potassium thiomethoxide (K+−SCH3) in DMSO? Compound H will react faster than J. This is an SN2 reaction, and iodide is a better leaving group than bromide. Weaker bases typically make better leaving groups, and the pKa of HI = −10 while the pKa of HBr = −9. (2) (9 points) Of compounds K and L, which reacts faster with methanol? Compound L will react faster than K. This system will be a competition between SN1 and E1 processes, which both have formation of the carbocation as the rate‐determining step. Since formation of the carbocation in an endothermic step, by the Hammond Postulate, features that stabilize carbocations will stabilize the transition state for this step, and thus, speed the reaction. The key difference between K and L is the cyano group on the phenyl ring, which is electron‐withdrawing by a resonance effect. This effect destabilizes the positive formal charge of the benzyllic carbocation: CHEM 346 – Organic Chemistry I – Fall 2014 – Quiz #3 – Solutions Key Page 8 of 8 (3) (9 points) Of carbocations M and N, which is more stable? (Note: I want to see some sort of drawing or di‐
agram in your explanation.) Carbocation N is more stable than carbocation M. Carbocations substituted with more R groups are more sta‐
ble due to hyperconjugation, where filled σC–H orbitals can align with, overlap with, and donate electron densi‐
ty into with the vacant unhybridized p orbital of the carbocation. These interactions help stabilize the cation, and are not possible for M, which has no bonds with orbitals capable of overlapping with the p orbital of the cation. Problem IV. Synthesis (12 points). Provide a synthetic route—i.e, a sequence of reactions—to produce com‐
pound Q using propyne (P) as the starting material and any other reagents you wish that contain two carbons or fewer.